You're just a step away from a reef, you know.

If you keep clams and anemones, you will need reef lighting, which will be expensive for your tank. You'll also need to pay attention to some water parameters (e.g. calcium) that don't matter as much with fish. The advantages of a FOWLR that big is that you will be able to keep some wonderful fish that would be too big or crowded in the 120, and some beauties that would munch on a reef.
Your tank will be a great upgrade for your tangs and foxface, as they mature. You will also be able to keep other fish that would quickly outgrow your 120. The 265 will be a nice size for fish like larger angelfish, or the less aggressive triggers (like blue throat, Indian - one of my favorites, or cross-hatch). You can keep fish that would be cramped in your 120, that will swim and act more normally in the greater space of the 265.
Some fish on my "must have" list, were I to keep a tank that size:
-Emperor angelfish (I had one in my 210 - a 265 would be even better!)
-Australian harlequin tuskfish - one of the best fish ever for color, temperament, and with blue teeth! (not invert safe though)
-Mimic tang - actually fine for a 120, but one of the most under-rated fish, IMO
-Dalmation or black color morph dogface puffer - the best puffer for temperament and size, IMO- and the Solomon Island color morphs are really elegant looking
If you don't invest in lights, and don't overstock your tank, your up front costs for fish will be high (though less than for corals) but the cost of maintaining your tank will be relatively low due to less electricity and bulb replacement. You also are unlikely to need a chiller. If you don't overstock the tank, and keep either a refugium or plenty of live rock and macro in the tank, you may be able to get by with less frequent water changes than a reef (though DON'T skimp on water quality, and invest in the best protein skimmer you can afford.) I'd light the tank with T5 or power compact fluorescents - get a light canopy used, in good condition, from someone upgrading their lighting for a reef, or getting rid of a relatively new system.
Good luck, I'm jealous!
If you want clams, and the fish you want for the 265 prove incompatible with inverts, can you keep the 120 with your smaller fish, and make it a clam and anemone, invert-centric reef?